The Pros and Cons of Using Larger Femoral Heads in Total Hip Arthroplasty

A Review Paper

Pranav Rathi, MBBS, MS, Gavin C. Pereira, MBBS, FRCS (Eng), FRCS (T&O), Mauro Giordani, MD, and Paul E. Di Cesare, MD

Twenty-five percent of all revisions are performed for instabil- Abstract ity or dislocation.3 With the introduction of improved bearing surfaces for total hip arthroplasty (THA) has come Total Hip Arthroplasty Stability a reintroduction of larger femoral heads with the and Jump Distance promise of reducing the rate of hip instability Jump distance (JD) is the femoral head center translation dis- and increasing hip range of motion (ROM). The tance required for a head to dislocate from a socket (Figure 1). size of femoral heads used for THA ranges from Prosthetic hips with less JD are more likely to dislocate than 22 to 40 mm, and even larger heads are used hips with more JD. Sariali and colleagues5 found that JD varies for hip resurfacing. With accurately positioned according to cup abduction angle (angle of rotation around components, larger heads reduce the hip insta- anterior-posterior axis of pelvis), cup anteversion angle (angle bility rate and theoretically increase hip ROM. of rotation around cranial-caudal axis of pelvis), femoral head However, for any given bearing surface, the vol- diameter, and cup center offset. umetric wear rate is higher for larger heads than Cup center offset is the shortest distance from the center of for smaller heads, which potentially jeopardizes the head to the opening plane of the cup (Figures 2A-C). The the long-term survival of these reconstructions.AJO offset is positive when the head center lies outside the opening In this article, we review the evidence for use of larger femoral heads with respect to stability, ROM, impingement, wear rate, bearing surfac- Figure 1. Jump distance (JD): distance of translation of femoral es, and future directions. head center required for head to dislocate from cup. Prosthetic hips with less JD are likelier to dislocate more easily than those DO NOTwith more JD.COPY ecent advances in total hip arthroplasty (THA) bearing materials, such as cross-linked polyethylene, ceramics, R and metal, have reduced the rate of bearing wear. The prospect of decreased wear (attributed to improved bearing materials) and the perception of a lowered rate of hip dislocation have prompted surgeons to use larger femoral heads for THA. The validity of this approach has not been proved. Femoral heads ranging from 22 to 40 mm are available for THA, and even larger heads are used for hip resurfacing. Both the type of material and size selected for femoral heads may affect dislocation rate, linear wear, and range of motion (ROM). In this article, we review the evidence for use of larger femoral heads with respect to stability, ROM, impingement, wear rate, bearing surfaces, and future directions. Dislocation rates are 0.5% to 10% for primary THA1 and 10% to 25% for revision THA.2 Up to one-third of hip dislocations become recurrent and require revision surgery.3 Every closed reduction episode increases hospital costs—by 19% for an uncomplicated THA—and by 148% for a revision THA.4

Authors’ Disclosure Statement: The authors report no actual or potential conflict of interest in relation to this article. www.amjorthopedics.com August 2013 The American Journal of Orthopedics® E53 The Pros and Cons of Using Larger Femoral Heads in THA P. Rathi et al

A B C

Figure 2. Cup center offset: neutral (A), positive (B), and negative or inset (C).

AJO DO NOT COPY Figure 3. Jump distance (JD) decreases as cup abduction angle increases from 25° to 70°. plane of the cup, which is the case when the cup is not a full It is only partly true that JD increases with femoral head hemisphere or when the acetabular liner is offset to increase diameter. The equation highlights that JD depends not only the amount of material medially in the polyethylene liner. The on femoral head size, but also on orientation of the implanted offset is negative (and is called an inset) when the head center lies cup and on cup offset. The position of the implanted cup in inside the opening plane of the cup. turn has 2 variables that affect JD: abduction angle and ante- The surgeon controls the variables of cup abduction angle, version angle. cup anteversion angle, and femoral head diameter, and JD is For a constant anteversion angle and a constant femoral head calculated as follows: size, JD is inversely related to the abduction angle (Figure 3). For example, for a 32-mm head, JD decreases about 0.25 mm π offset − arcsin JD = 2R × 2 θ ()R for each 1° increase in abduction angle. []2 With respect to constant anteversion angle and constant where R = femoral head radius, θ = planar cup inclination abduction angle, JD is directly related to femoral head size. angle, and offset = cup center offset. The planar cup inclina- However, this advantage of increased JD is reduced if the cup tion angle is given by: is implanted with an increased abduction angle, ie, vertical cup placement. ϴ = arctan [ tan α × cos β ] For an acetabular component at an abduction angle of 30°, JD increases 0.5 mm for each 1-mm increase in head diameter; where α = abduction angle of cup, and β = anteversion angle at an abduction angle of 45°, JD increases 0.4 mm; and, at an of cup. abduction angle of 60°, JD increases 0.25 mm. The JD increase

E54 The American Journal of Orthopedics® August 2013 www.amjorthopedics.com The Pros and Cons of Using Larger Femoral Heads in THA P. Rathi et al

Figure 4. Jump distance decreases as cup center offset increases.

that occurs with larger head diameter depends on the abduc- femoral head offset at the start. For a 1-mm increase in head tion angle/inclination of the cup, and much of that advantage offset, JD is decreased by 0.92 mm. Generally, in large-diam- can be lost when the cup is inserted at a higher than desired eter head–cup couples (head size, >38 mm), the head offset abduction angle. For example, in a cup inserted at 45°, JD is increased by about 3 mm. In these cases, JD is 2.76 mm increases from 14.1 mm for a 36-mm head to 15.8 mm for a less than the JD for a corresponding hemispherical cup design. 48-mm head—a difference of 1.7 mm (12%), which drops to When comparing JD for a 32- to 40-mm head diameter, one 0.13 mm (1.4%) for a cup abduction angle of 60°.AJOmust consider the acetabular couple and the respective femoral JD also varies directly with the anteversion of the acetabu- head offset because each may have a similar JD. This is why lar component, but this variation has less effect per degree use of very large heads results in a smaller increase in JD than of position change than the abduction angle.5 This is because expected, and may be why similar dislocation rates are found the anteversion angle is a low-value angle (15°-25°) from the in comparisons of 32-mm heads and larger heads.5 start, and consequently, the trigonometric value of this angle The biomechanical basis for the increased stability of larger is almost negligible. femoral heads in THA has received support from clinical data. DOThe other variable that affects NOTJD is cup center offset. This According COPYto an analysis of 42,987 primary THAs from 1987 relationship is inverse, ie, as offset increases, JD decreases. to 2000 (The Norwegian Arthroplasty Registry), the rate of Increasing the offset causes the center of rotation to lay outside revision surgery for instability was lower for patients with the opening plane of the cup, thereby making the cup shal- larger femoral heads (30-32 mm) than for patients with smaller lower and increasing susceptibility to dislocation (Figure 4). femoral heads (≤28 mm).7 In a separate analysis of a polished, Decreasing the offset, or having an inset (as in a constrained tapered, cemented stem (Exeter; Howmedica Osteonics, Caen, liner), can increase the risk for impingement. However, Bunn France) and an all-polyethylene, cemented cup (Exeter; How- and colleagues6 studied ROM in a computer-generated pelvis medica Osteonics, Caen, France) used in THAs with 26-, 28-, with a Secur-Fit Max Femoral Hip Stem (Stryker Orthopaedics, and 30-mm heads (same registry), the reoperation rate was Mahwah, New Jersey) and a Trident Acetabular Cup System higher for patients with 26-mm heads (because of dislocation) (hemispherical with 0 cup center offset; Stryker Orthopae- than for patients with 30-mm heads.7 The higher reoperation dics, Mahwah, New Jersey) and concluded that bony impinge- risk after dislocation persisted even after adjusting for several ment—not implant impingement—reduced ROM. potential confounders. It should be noted that this registry did As femoral head size increases, the polyethylene liner thins not capture hip dislocations for which there was no reopera- to accommodate it, but only to a point (the liner has a certain tive repair. minimum thickness). Therefore, femoral heads larger than In 2005, the Mayo Clinic reported on 21,047 THAs per- 38 mm require larger acetabular components so that a liner of formed between 1969 and 1999 to determine the effect of at least minimum permissible thickness can be used. In theory, femoral head diameter on risk for dislocation.8 Many factors the surgeon can decide to ream for a larger cup to accommo- were stratified and adjusted to isolate head diameter as a single date the larger head, creating more acetabular bone loss. To variable. Results showed that larger heads were associated with reduce this bone loss, manufacturers pair large femoral heads lower risk for hip dislocation. The dislocation risk was highest with acetabular components that are truncated hemispheres for 22-mm heads, intermediate for 28-mm heads, and lowest of 165°, instead of 180°. These pairings result in a positive for 32-mm heads. To determine the rate of dislocation 3, 6,

www.amjorthopedics.com August 2013 The American Journal of Orthopedics® E55 The Pros and Cons of Using Larger Femoral Heads in THA P. Rathi et al

Figure 5. Larger femoral head increases hip range of motion from 111° to 135°. Increased impingement-free range of motion should reduce hip instability rate.

12, and 18 months after THA, investigators analyzed 247,546 the prosthetic femoral neck and the cup liner or shell, and procedures performed between 2004 and 2009 (National bone-to-bone contact can occur between the femur (greater Joint Registry of England and Wales).9 There was a statistically trochanter) and the pelvis.15-17 significant increase in use of large-diameter femoral heads In prosthetic hips, a head–neck ratio of less than 2.0 re- (≥36 mm), from 5% in 2005 to 26% in 2009, alongAJO with a con- duces ROM and significantly increases risk for impingement. comitant increase in use of the posterior approach, from 34% An increase in head diameter increases ROM primarily by in 2004 to 57% in 2009. The compiled data revealed a signifi- increasing the head–neck ratio (Figure 5). The head–neck ratio cant reduction in dislocations associated with larger femoral depends on head size, femoral neck geometry, and use of a heads, even when implanted through a posterior approach. skirt on the femoral head.18-20 Use of a larger head also ensures In another recent retrospective review, the 0.05% disloca- an acceptable head–neck ratio irrespective of neck geometry tion rate found for 2020 primary THAs with larger femoral and taper modes. headsDO (>36 mm) was lower than NOTthe rate (0.8%) for 1518 pri- D’Lima COPYand colleagues14 reported a nonlinear relationship mary THAs with smaller femoral heads (≤32 mm); the differ- between femoral head size and hip ROM. There was more ence was significant P( <.001).10 improvement in hip ROM when femoral head size increased In summary, larger diameter femoral heads reduce the dis- to 26 mm (from 22 mm) than when it increased to 32 mm location rate in primary THA and revision THA, but this advan- (from 28 mm), despite the increases being the same (4 mm). tage becomes less well-defined for femoral heads larger than The study results showed a plateau in the advantage of using 38 mm. There appears to be a sound biomechanical basis for larger femoral heads. these results, and the clinical outcome data substantiate these Cup orientation and femoral head size also affect hip ROM. findings. Larger femoral heads can be used advantageously Maximal impingement-free ROM occurred with cup abduction in patients at increased risk for hip dislocation; however, the angles between 35° and 45° and cup anteversion angles between advantage of larger heads can be lost when the acetabular com- 0° and 10°.21 Higher cup abduction angles result in increased ponent is not positioned correctly. hip flexion, extension, and external rotation,14 though this is associated with decreased hip stability. Increasing femoral head Range of Motion size can result in increased prosthetic impingement-free ROM. ROM after THA depends on many factors, including patient However, if the abduction angle of the cup is increased, the factors (eg, obesity, preoperative functional status), physio- larger head will contribute little to increased hip stability and therapy adherence, surgical factors (eg, approach), component may result in accelerated wear.22 placement, and implant design (eg, femoral head diameter, Clinical evidence of the effect of head diameter on actual head–neck ratio, neck length).11-14 Increased ROM, however, ROM is not as convincing as the evidence for the decreased may come at the cost of hip stability, with increased risk for rate of dislocation with larger diameter femoral heads. Mont impingement along with increased bearing wear. Prosthetic and colleagues23 performed gait analyses to compare hip re- impingement determines the functional endpoint of stable surfacing (large-diameter femoral heads), standard THA, os- ROM after THA. In THA, impingement can occur between teoarthritic hips (native), and normal hips. Patients with hip

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A B

Figure 6. Joint center moved medially compared with native hip (A), minimizing joint forces (B). AF: abductor lever arm, BW: body weight, JRF: joint reduction force.

resurfacing walked faster and with a gait comparable to indi- stability. The effect on viduals with normal hips while those with standard THA walked hip ROM, however, is slower. However, the resurfacing and THA groups had simi- less clear. In THA, ROM lar hip abduction and extensor movements. The authors sug- increases with larger gested this was attributable primarily to use of larger diameter head diameters primar- femoral heads, which they postulated restore the kinematics ily because of increased of artificial hips more closely to that of normalAJO native hips. stability and reduced These findings were corroborated in a clinical study in which impingement. Although digital photographs and bony landmarks were used to assess this does not produce an hip ROM at a minimum 1-year follow-up in patients with ei- obvious clinical advan- ther THA (larger diameter femoral heads) or hip resurfacing.24 tage in many patients, it The authors noted that the arc of motion in the THA group may normalize gait bio- (~20°, primarily in hip flexion and external rotation) was mechanics and improve largerDO than the arc in the hipNOT resurfacing group (flex- activities ofCOPY daily living. ion, 0-112.4; external rotation, 0-36.7). They postulated Figure 7. Larger acetabular compo- that this was due to the larger head-to-neck diameter ratio. Wear nent combined with larger femoral head lateralizes hip center. A, B, Hip ROM was examined in THA with 26- and 32-mm fem- Femoral head size can and C are the hip centers of se- oral heads to determine the effect of these heads on patients’ affect polyethylene quentially larger heads paired with ability to perform selected activities of daily living (eg, put- wear. Wear is a multi- corresponding large cups. ting on and removing trousers and socks, cutting toenails).25 factorial phenomenon Many patients in the 26-mm group adopted a compensatory influenced by head size position of lumbar flexion with hip flexion plus knee exten- and many other implant factors (properties and composition sion, whereas a majority of the patients in the 32-mm group of articulating parts; polyethylene quality and configuration) used a more normal mode of hip flexion with knee flexion to and patient factors (age, sex, activity level, weight).27 perform these selected activities. The center of hip rotation affects wear through changes Although many studies of THA with larger femoral heads in joint reaction forces.27 When the joint center is moved me- have demonstrated increased ROM, there is almost an equal dially, inferiorly, and anteriorly, joint forces are minimized. number of studies showing otherwise. Le Duff and colleagues26 This position increases the abductor lever arm and the mo- reported no difference in hip ROM between hip resurfacing ment-generating capacity of the abductors in turn decreas- (large femoral heads) and THA, even after separating the co- ing the external moment that needs to be balanced by the hort into 2 head-size groups (<40 mm, ≥40 mm). The authors muscle forces by bringing the hip center closer to the center concluded that, for most patients, prosthetic design is unlikely of gravity. Consequently, when the joint center is moved su- to be a factor limiting hip ROM after surgery provided that periorly, laterally, and posteriorly, as is the case with a dys- cup position is adequate. plastic hip with cup placed in the false acetabulum, joint In summary, biomechanical and clinical studies have shown forces and moments are stronger. Therefore, more me- that use of larger diameter femoral heads in THA increases hip dial placement of the hip center affects wear beneficially,27

www.amjorthopedics.com August 2013 The American Journal of Orthopedics® E57 The Pros and Cons of Using Larger Femoral Heads in THA P. Rathi et al by reducing joint reaction forces (Figures 6A, 6B). For larger metric wear with the larger diameter heads. Lachiewicz and femoral heads (>38 mm), there is often a head offset of about colleagues36 cautioned against using larger femoral heads in 3 mm as the cup design corresponds to a truncated hemisphere young active patients at low risk for instability, as the longer of 165°. This in effect lateralizes the hip center, increasing joint term sequelae of increased volumetric wear are unknown. reaction forces and theoretically increasing wear (Figure 7). Although the wear characteristics of improved polyethylenes Volumetric wear is a measure of the absolute amount of ma- have reduced the volume of wear particles to an almost unde- terial removed from the bearing surface. The increased contact tectable level, larger diameter heads still cause more wear than area and sliding distance of larger heads result in increased smaller diameter heads and it is unclear if this amount of wear volumetric wear. This can be represented by the following will translate into osteolysis and premature implant failure. simple cylindrical formula: In conclusion, the biomechanical and clinical data support V = (π)R2W the finding of increased stability and increased ROM with use In the above V is the change in the volume of the polyethylene of larger diameter femoral heads—advantages that come at bearing (volumetric wear); R is the radius of the femoral head; the cost of increased bearing surface wear, which may affect and W is the measured linear wear. This formula demonstrates implant longevity. More clinical studies are needed to under- that for any given amount of linear wear, volumetric wear stand the clinical long-term effects of larger heads used with increases exponentially with increases in the radius of the various bearing surfaces. Until these studies are completed, bearing (head diameter). In a retrieval study, for each milli- surgeons should use large femoral heads cautiously in young meter increase in head diameter, there was a volumetric-wear or active (high-demand) patients and in patients at low risk for increase of 6.3 mm3 per year.28 instability. Large femoral heads provide an increased margin Clinical studies found rates of linear wear, osteolysis, and of error for stability in terms of component placement during implant loosening of polyethylene against 32-mm femoral surgery, but this advantage can be negated if the acetabular heads to be equivalent to or greater than those in heads with a component is malpositioned.35 smaller diameter.28 Livermore and colleagues29 reported higher volumetric wear rates for 32-mm femoral heads as well, in Dr. Rathi is Clinical Fellow in Adult Reconstruction, University of comparison with 28-mm heads. In another study, there was California Davis Medical Center, Sacramento. Mr. Pereira is Assistant significantly (P<.001) more volumetric wear in THAs with Professor, Department of Orthopedic Surgery, and Consultant, Adult 3 Reconstructive Services, University of California Davis Medical Cen- 32-mm femoral heads (136 mm /y) than in THAs with 28-mm ter, Sacramento. Dr. Giordani is Associate Professor, Department of 3 30 AJO heads (51 mm /y). Orthopaedic Surgery, Director, Adult Reconstruction Fellowship, and The influence of head diameter on midterm (range, Chief, Adult Reconstructive Services, University of California Davis Medical Center, Sacramento. Dr. Di Cesare is Director, Total Joint 2-12 years) cumulative revision rates was studied by Marston Arthroplasty Service, New York Hospital, Queens. 31 32 and colleagues in 1996 and by Kesteris and colleagues in Address correspondence to: Pranav Rathi, MBBS, MS, University 1998. They reported on outcomes of 413 and 1660 THAs of California Davis Medical Center, 4860 Y St, Suite 3800, Sacra- (all conventional polyethylene), respectively, and found sur- mento, CA 95817 (tel, 916-596-8225; fax, 916-734-7904; e-mail, vivalDO unaffected by head size. However,NOT after follow-up of the [email protected]). COPY 1660-patient of the Kesteris cohort by Tarasevicius 10 years Am J Orthop. 2013;42(8):E53-E59. Copyright Frontline Medical Com- munications Inc. 2013. All rights reserved. later at 21 years after surgery, it was found that the 32-mm cohort had a higher risk for revision. For larger femoral heads with either cross-linked or vita- References 1. Sariali E, Leonard P, Mamoudy P. Dislocation after total hip arthroplasty min E–impregnated polyethylene, long-term wear rates and using Hueter anterior approach. J Arthroplasty. 2008;23(2):266-272. revision risks are yet unknown. 2. Alberton GM, High WA, Morrey BF. Dislocation after revision total hip Biomechanical studies have found that conventional poly- arthroplasty: an analysis of risk factors and treatment options. J Bone Joint Surg Am. 2002;84(10):1788-1792. ethylene and cross-linked polyethylene have different wear 3. Bozic KJ, Kurtz SM, Lau E, Ong K, Vail TP, Berry DJ. The epidemiology of behavior. Hip simulator studies have shown extremely low revision total hip arthroplasty in the United States. 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